First Published in 1998. This is Volume IV in the Sociology of Behaviour and Psychology series. In the preparation of this book two aspects of the work of the Institute of Human Relations are illustrated. The first is that of co-operative research. It has been co-operative not only in the that sense that five authors amid three collaborators have contributed to it, but more. fundamentally, they have pooled their ideas and materials, and criticisms. Also, the procedure of inquiry which is well known but seldom used in the social sciences has also been used. It begins with a problem or a group of problems that are real in the experiences of daily life, then after definition a theory is tested. The problem of aggressive behaviour is here advanced one step' along this road which all· social inquiry that aspires which has consisted partly in a more ·systematic- formulation and further elaboration of the Frustration-Aggression hypothesis which had already— been stated by·Freud and others